Making Sense of- Variation in Fabric Weight (GSM/OzY2) Differences from location to location – Logic and Reason – Based on ASTM D 3776

Purpose of this Document
( For those of you who dont want to read through the whole text, please scroll to the last paragraph, where I have explained all of this in simple words)
To give a logical explanation for variation in fabric weight ( GSM) when measured in different locations.
To understand this one should understand the role atmospheric moisture called Relative humidity that plays a significant role in increasing or decreasing the weight of textile fabrics. In scientific parlance a term called Moisture regain is used to explain the ability of different fibers to absorb water from the atmosphere and retain it in them. ( See table 1 below that shows the max moisture regain of different fibers at the standard condition for testing textiles which is Temp of 20+/- 1 Deg C and Relative humidity of 65 +/- 2% RH).
What is Relative humidity  and why is it relative – Relative Humidity of air is  the ratio of the pressure of water vapor present to the pressure of saturated water vapor at the same temperature. Saturated is 100%, the pressure of Air with 100% water vapor is 100% Humidity, as the water vapor content in air reduces the pressure reduces and this is calculated relative to the 100% pressure and expressed as a % of Humidity, since it is relative to 100% water vapor , its called relative humidity.
Absolute humidity —the total mass of water vapor present in a unit volume of air.
Humidity —the condition of the atmosphere in respect to water vapor.
Moisture regain —the amount of water re-sorbed by a dried textile material at specified equilibrium conditions of temperature and humidity, compared to the mass of the dried material. This is normally calculated at standard atmosphere which is Temp of 20+/- 1 Deg C and Relative humidity of 65 +/- 2% RH. One must keep in mind that the Moisture regain will reduce with RH less than 65% or be more where the RH is above 65%.
Standard moisture regain —the moisture regain of a material at equilibrium with the standard atmosphere for testing textiles. The standard atmosphere for testing textile is accepted as a Temp of 20+/- 1 Deg C and Relative humidity of 65 +/- 2% RH. Below table gives standard moisture regain values for various textile fibers.Do remember that in order for a fabric to confirm to the below given moisture regain values it must be dry prior to conditioning. If the materials is removed form the Stenter or compactor where steam is applied, such material should not be weighed for GSM. These materials must first be dried at 50 Deg C until it is bone dry and then conditioned for a minimum period given in table 2 to enable it to regain moisture and come to equilibrium at standard atmosphere.
Moisture regain Table
Fiber Minimum Conditioning Period 
Conditioning Times

All weighing tests should be made in the standard atmosphere for testing textiles (20 +/-1°C , 65 +/- 2 % RH , after the specimens have been conditioned in the same atmosphere. 

Where weightings are conducted  without conditioning the textile material for a minimum period mentioned in the above table . Such results may not correspond with the results obtained after testing adequately conditioned specimens in the standard atmosphere for testing textiles. This is the main reason for the difference.
In case of a dispute arising from differences in reported test values when using Test Methods D 3776 for acceptance testing of commercial shipments, the purchaser and the supplier ( or the mill and 3rd party lab) should conduct comparative tests to determine if there is a statistical bias between their laboratories. Competent statistical assistance is recommended for the investigation of bias. As a minimum, the two parties should take a group of test specimens which are as homogeneous as possible and which are from a lot of material of the type in question. The test specimens should then be randomly assigned in equal numbers to each laboratory for testing. The average results from the two laboratories should be compared using student’s t-test for unpaired data and an acceptable probability level chosen by the two parties before testing is begun. If a bias is found, either its cause must be found and corrected or the purchaser and the supplier must agree to interpret future test results in the light of the known bias.
Another reason for the difference is the standards say take 10 Circles each 113 Dia meter ,. The Circles must be cut in a diagonal manner to cover various length and width yarns , and then weight them, but almost all the people only use one circle and determine the weight. – Natural fibers are highly variable and basing any result on one specimen will only give errors . Remember the larger the population the larger the sample size should be.
Just to make you understand more clearly , lets say I am asked to conduct a survey as to how many men in India are bald. I take a sample size of 2 men and both are found bald, so my conclusion will be 100% of all Indian males are bald. Which we all know is not true. This same logic applies to GSM calculation also. To cover for the variations that may occur across the length and width of the fabric we take 10 circles and not one.
Another consideration is total sampling, if the lot size is 1000 meters , or 100,000 meters the sampling cannot be just the 10 circles, we need to increase the specimens as per the lot size.

Remember the larger the population  larger the sampling size should be.

OK now to explain this in simple words :
When I am sitting in a place where the air is very wet say like Goa or Kerala and you are sitting in Delhi where the air is very dry. When I weigh my cotton or viscose or Wool , it will show a higher weight than when you weigh it in Delhi. So we both fight. To avoid this fighting, we must exchange some standard cotton and Standard viscose fabric and any other fiber that we use regularly , every month and establish what's the difference we see in the weighing every fortnight and establish the bias between our weighing and agree to that bias as the difference that will occur between our two weighing.
This is what we say agree to disagree and also establish the amount of disagreement.
For for details please read ASTM D 123, 1776, 1909 and 3776
Acknowledgements to ASTM International.